The US military is taking the blame for an airstrike in Afghanistan over the weekend that left at least six civilians dead — one mother and five children.

NATO officials confirmed on Monday that an airstrike launched late Friday in the Sangin district of Helmand Province, Afghanistan resulted in the death of at least half a dozen civilians. The news was first revealed by Governor Muhammad Gulab Mangal of Helmand who, according to the New York Times, called the US regional commander on early Monday to ask for an explanation.

“We expressed regret over the incident, and we’re investigating to determine how this happened,” Lt. Col. Stewart Upton, a spokesman for the US military in Helmand, tells the Times.

“We are deeply saddened by any civilian death and particularly regret an incident where civilians are killed,” he adds.